Mike Reiss is reporting that OLB Shawn Crable, DL Kade Weston and WR Wes Welker will start camp on the active, but physically unable to perform list. While this list isn't set in stone, placing Welker on the list is most likely because he's returning from an injury and isn't anything to worry about. Crable and Weston on the list is a little more interesting. All three players can be taken off the active/PUP list at any time, but it's curious as to why each player was put on the list in the first place.
Welker is on for obvious cautionary reasons, but this seems to stall the hopes of Welker going 100% entering camp.
Crable is questionable, having spent the past two seasons on the injured reserve. However, activities haven't started yet, so there's no real need to worry. If Crable doesn't show up on the field when camp really starts, that should be more cause for concern- and Crable can most likely kiss his roster spot good bye. I think Crable's spot on the list has to do with the team devoting two pre-seasons to him. By active/PUPing Crable, it allows the Patriots to tryout another player, just in case Crable doesn't work out.
Weston suffered from a knee injury in 2008, which could attribute to his placement on the active/PUP list. Like Crable, Weston's spot might not mean anything and could just be an early formality, but it could also mean he's recovering from the injury still and the Patriots want to take it slowly. I believe that Weston's on the list because he missed a lot of the pre-season training due to his inability to get a work visa. It's not a knock against Weston, but he may need to catch up and placing him on the PUP allows him to catch up without him worrying about a roster spot.
Don't worry about this because none of these players have really taken the field yet. Once camps starts, we'll see how serious these placements are.
EDIT: Well, here's an update. Shalise Manza-Young of the Boston Globe is reporting that Crable is actually injured, but it's unknown how long he'll sit out for. Oh brother...